1. Field of the Invention
The present invention relates to a radio communication system suited to a radio local area network in which a plurality of terminals are connected by radio.
2. Description of the Related Art
As computers have become highly functional, it has been frequently performed that a plurality of computers are connected to form a local area network (LAN) in order to share files and data and to transfer data and electronic mail. In a conventional LAN, computers are connected by wire with the use of optical fibers, coaxial cables, or twisted pair cables.
In such a wired LAN, since connection work is required, it is difficult to form a LAN easily. In addition, cable wiring is troublesome in a wired LAN. Therefore, a radio LAN has drawn attention as a system which releases the user from wiring required for the conventional wired LAN.
A radio LAN has been proposed in which data communication is performed by the code division multiple access (CDMA) method with the use of spread spectra. In the CDMA method, data to be transmitted is multiplied by a pseudo noise (PN) code to spread the spectra of the data to be transmitted. The sent data with its spectra being spread is demodulated by multiplying the PN code which is the same as that used in the transmission side. The CDMA method features a high privacy in communication and a strength against interference.
In recent years, information has been distributed in multimedia and large data in size, such as image and sound data, has been handled in many cases. Therefore, it is demanded that a radio LAN provide a higher transfer rate so as to transmit large data such as image and sound data. In spread spectrum modulation, when data is transferred at a high rate of about 30 Mbps, for example, a bandwidth of more than 300 MHz is required. Such a wide bandwidth cannot be obtained in the current frequency allocation. In addition, it is difficult to perform communication with such a wide bandwidth.
In spread spectrum transfer, a synchronization acquisition time is required in demodulation in order to achieve phase matching between the sent data code and a code generated by a receiver for demodulation. To implement this, a bit string for synchronization is inserted into each packet in spread spectrum transfer for high-speed synchronization acquisition. Due to this bit string for synchronization, the number of bits other than those of effective data increases.
Accordingly, an object of the present invention is to provide a radio communication system which allows high-speed data transfer and which allows the timing of transmission and receiving to be set without providing a number of synchronization bits.
The above object is achieved in one aspect of the present invention through the provision of a radio communication system including: a plurality of radio communication terminals for data communication and a radio communication control terminal for controlling radio communication, the radio communication control terminal including: transmission means and receiving means performing data transmission and receiving by the OFDM method, and synchronization-code generating means for generating a code sequence for synchronization acquisition, and each of the plurality of radio communication terminals including: transmission means and receiving means performing data transmission and receiving by the OFDM method; synchronization-code detecting means for detecting the code sequence for synchronization acquisition; and timer means set by the synchronization-code detecting means, wherein data is modulated by the OFDM method and the data is multiplexed by the TDMA method with a frame structure having the specified number of symbols, between each of the plurality of radio communication terminals and the radio communication control terminal, the radio communication control terminal transmits the code for synchronization acquisition to the plurality of radio communication terminals in each frame, and each of the plurality of radio communication terminals receives the code for synchronization acquisition, sets the timer means according to the receiving timing of the code for synchronization acquisition, and sets the transmission timing and the receiving timing with the timer means being used as the reference.
The foregoing object is achieved in another aspect of the present invention through the provision of a transmission apparatus including: an OFDM signal generating section; and synchronization-code generating means for generating a code sequence for synchronization acquisition, wherein a signal in which data is modulated by the OFDM method and the data is multiplexed by the TDMA method with a frame structure having the specified number of symbols, and the code for synchronization acquisition are transmitted.
The foregoing object is achieved in still another aspect of the present invention through the provision of a receiving apparatus including: an OFDM signal receiving section; and synchronization-code detecting means for detecting a code sequence for synchronization acquisition, wherein the code for synchronization acquisition is received and the receiving timing is set according to the code.
The foregoing object is achieved in yet another aspect of the present invention through the provision of a transmission method including the steps of: generating an OFDM signal; and generating a code sequence for synchronization acquisition, wherein a signal in which data is modulated by the OFDM method and the data is multiplexed by the TDMA method with a frame structure having the specified number of symbols, and the code for synchronization acquisition are transmitted.
The foregoing object is achieved in a further aspect of the present invention through the provision of a receiving method including the steps of: receiving an OFDM signal; and detecting a code sequence for synchronization acquisition, wherein the code for synchronization acquisition is received and the receiving timing is set according to the code.